Implementations of the claimed invention generally may relate to schemes for canceling echoes or reflections and, more particularly, to schemes for canceling echoes using adaptive filters.
When using digital communication techniques in terrestrial communication, fairly sophisticated equalization circuits may be needed to compensate for severe and/or long delayed reflections (e.g., echoes). For single carrier protocols, for example, the equalizer may need to compensate for pre-echoes and/or post-echoes (which may also be called reflections). As used herein, “pre-echoes” may be conceptualized as those signals of interest arriving earlier than an expected/reference time (e.g., due to multipath effects), and “post-echoes” may be conceptualized as those signals of interest arriving later than an expected/reference time (e.g., due to multipath effects). One possible approach for handling such echoes/reflections may be to build an adaptive filter equalizer, with two branches (i.e., feed forward and feedback branches) to compensate for pre-echoes and/or post-echoes.
FIG. 1 illustrates a typical adaptive filter 100 according to such an approach. Filter 100 may include a feed forward branch 110, a combiner 120, an error estimator 130, and a feedback branch 140. Combiner may combine the outputs of branches 110 and 140, and error estimator 130 may produce an estimated error between its output and a desired or ideal output without any echo. This estimated error may be used to adjust the coefficients (e.g., characteristics) of branches 110 and 140 to minimize the error (e.g., to adaptively filter the input signal).
The length of feed forward branch 110 may be based on an expected worst case delay that is associated with a pre-echo. Similarly the length of feedback branch 140 may be based on an expected worst case delay that is associated with a post-echo. The lengths of branches 110 and 140 (and hence their delays) may be different. In this manner, the maximum delays associated with branches 110 and 140 may be sufficient for filter 100 to cancel out the expected, worst case pre-echoes and post-echoes that occur in the input signal.
The fixed lengths of branches 110 and 140, however, may not be amenable to different communication channels, and may not be an optimal use of silicon resources on a device.